1. Field of the Invention
The invention relates to an intervertebral nucleus prosthesis for restoring mobility in the area of an intervertebral disc. This mobility is created physiologically by the nucleus, which can be seen as a mechanical joint component of the vertebral bodies.
2. Description of Related Art
Physiologically, the nucleus is formed by an essentially spherical, nonextendible, but deformable body inserted like a ball between two end faces of vertebrae, essentially at the center thereof, and allowing inclining, rotating and sliding movements.
The end faces of the vertebrae themselves are surrounded by concentric layers of fibers referred to as annulus and containing fibers which cross one another obliquely from one layer to the other.
In this arrangement, the nucleus, which is actually a deformable, but nonextendible capsule filled with a hydrophilic, jelly-like substance (mucopolysaccharide), itself moves backwards or forwards during forward or backward bending movement of the spinal column, and this movement of the nucleus is limited by the back and front fibers of the annulus and by different ligaments.
When the spinal column inclines to the side, the displacement of the nucleus stretches the fibers on the convex side. The displacement is limited by the intervertebral ligaments.
During a rotation of the spinal column, the intervertebral disc is acted upon by shearing forces.
The nucleus, which is a hydrophilic body, has a specific osmotic pressure in the state of rest. Under load, the nucleus loses water. The thickness of the intervertebral disc decreases. Rehydration occurs once the pressure is reduced again.
The pathological states which affect the reciprocal movements of the various vertebral bodies are essentially overloading or excessive and repeated strain or natural aging processes.
In the event of excessive, repeated or prolonged strain, rehydration does not take place, and, consequently, the pressure exerted by the nucleus on the fibers forming the various concentric layers of the annulus is unable to be built up, and this pressure, therefore, also no longer carries out its restoring function during the movements. This results in instability accompanied by arthrosis.
The object of the invention is to prevent this instability from developing by inserting a nucleus prosthesis, implantation of which can be carried out in vivo and with less traumatization than has so far been possible with known prostheses of this kind.
These known prostheses, the principles of which will be examined hereinbelow, harbor drawbacks which, with a knowledge of the reciprocal movements and the function of the nucleus during these movements, are understandable.
These drawbacks occur during both use and insertion of these known implants.
Initially, it was simply suggested that those intervertebral discs located in the traumatized area be removed. A bony graft was then inserted, partly with and partly without osteosynthesis.
The blocking of two adjacent vertebral bodies thereby obtained did, however, merely transfer the load onto the intervertebral discs of the immediately adjacent vertebral bodies, which were thereby placed under excessive strain and quickly suffered damage. The problem was only shifted without solving it.
A complete intervertebral disc prosthesis including both annulus and nucleus has also been suggested.
The main disadvantage of this practice is that it requires total removal of the damaged intervertebral disc and then insertion of the complete prosthesis between two adjacent vertebral bodies.
Each of the two phases of this operation requires severing of the common vertebral body ligaments, namely the front ligament (LVCA) and the back ligament (LVCP), which does not allow the natural movements of the spinal column to be restored again, and this, therefore, suffers a loss of reliability.